Parabolic reflector antennas of the general class to which this invention relates are customarily made in several different manners. For instance they may be spun from a material such as aluminum or formed on a hydroforming press. Each of these methods of making a dished reflector has its disadvantages. Spinning as a method for making a dished reflector requires that the reflector be made from a metal that can be drawn or spun such as aluminum. This is a limitation and often results in higher costs. Also it may be necessary to anneal the metal during the spinning process, thereby adding to the cost. With hydroforming, the reflector can be made of any metal. Martin U.S. Pat. No. 3,672,194 issued June 27, 1972 describes and illustrates a hydroforming apparatus for shape forming sheet elements in which a disc-like sheet element is clamped in a press and expanded using hydraulic fluid into a die element of appropriate contour. The expense of such equipment obviously adds to the cost of the product as well as the availability of the method.
Yet another method of forming dished reflectors has been to expand a membrane into a hardenable material, and maintaining the membrane in an appropriate expanded state until the material hardens (see for example Wilenius et al U.S. Pat. No. 3,251,908 issued May 17, 1966 and Bagby U.S. Pat. No. 3,337,660 issued Aug. 22, 1967). Fassnacht et al U.S. Pat. No. 3,184,210 issued May 18, 1965 describes and illustrates a method of constructing a dished reflector in which a collapsible bladder is expanded to provide a mold pattern and a casting material is then sprayed on the contoured surface of the bladder and cured to provide a cast reflector surface which is a duplicate of the image surface of the inflatable bladder. All of these methods require significant technical expertise on the part of the persons who are making the reflectors and, often, expensive and sophisticated apparatus.
Others have developed methods and apparatus for shaping of paraboloidal surfaces by applying pressure to the central portion of a disc-like sheet while the sheet is overlying a backing disc and secured by a peripheral clamp ring so as to cause the central portion of the workpiece to distend outwardly into a dome-shaped product. Often the sheet is made of metal and the pressure is applied pneumatically. U.S. Pat. No. 3,934,440 of Berg issued Jan. 27, 1976 and U.S. Pat. No. 3,572,071 of Semplak issued Mar. 23, 1971 as well as Clough Canadian patent No. 640,483 issued May 1, 1962 are representative of this approach. This approach is economically attractive, since a die to receive and form the expanded sheet is not required. As well, it has been found that where a circular disc is used, a uniform pressure, sufficient to substantially exceed the elastic limit of the sheet material and thus to permanently deform it, results in a surface which is generally paraboloidal to a sufficiently high degree of approximation to be useful as a parabolic reflector antenna. One of the main problems experienced with this approach however has been in the clamping of the edges of the sheet. If the clamping is not uniform, circumferential buckling or wrinkling of the sheet may result. The apparatus of Berg U.S. Pat. No. 3,934,440 requires a clamping mechanism which permits controlled slippage of the margin of the sheet with respect to the clamps. Clough Canadian patent No. 640,483 describes and illustrates a clamp mechanism which comprises a single, massive piston which moves upwardly, into position to clamp the edges of the sheet between the piston's upper surface and a cooperating die.
Other patents of general background interest describing other constructions of parabolic reflector antennas are U.S. Pat. No. 4,455,557 of Thomas issued June 19, 1984, Canadian patent No. 1,121,911 of Vines issued Apr. 13, 1982 and Canadian patent No. 716,197 of Richards issued Aug. 17, 1965.